1. Field of Invention
The invention relates generally to the field of wireless communications and more particularly to a method of and device for detecting the presence of a received data packet in a digital receiver.
2. Description of the Related Prior Art
Rapid growth in the portable communications market has pushed designers to seek low-cost, low-power, highly integrated solutions for the radio frequency (RF) transceiver in accordance with the IEEE 802.11a-1999 Part 11: “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications—High Speed Physical Layer in the 5 GHz Band which is incorporated herein by reference. As those skilled in the art will appreciate, an important aspect of any wireless receiver is the ability to detect the presence of data packets where the received signal may be degraded as a result of multi-path fading and/or the presence of noise.
A wireless radio receiver employing digital demodulation methods typically detects the presence of energy in order to start a timing machine to control the proper acquisition of the radio signal. This acquisition typically involves determining a radio frequency preamble or known ‘signature’ signal.
Acquisition involves applying the input signal to a filter “matched” to the original RF signature and examining the matched filter output for a “peak” to signify that a signal with a valid signature was detected. As will be appreciated by those in the art, certain radio frequency applications perform signal processing operations by separating radio frequency signals into signal components representing in-phase (I) and quadrature (Q) channels (I/Q channels). If the RF signal is demodulated to complex I/Q baseband then the matched filter must also be complex I/Q at baseband. This process is defined as “complex matched filter correlation” or “correlation” for short.
If the radio signal undergoes rapid automatic gain control (AGC) fluctuation during the acquisition process, then the quality of the correlation peak will deteriorate and the determination of a valid signature may be compromised. As those in the art will appreciate, an AGC circuit is a circuit by which gain is automatically adjusted in a specific manner as a function of a specified parameter, such as received signal strength (RSSI).
Past solutions normalize the baseband I or Q signals by dividing by an estimate of the AGC gain in a receiver line-up. The problem with this approach is that it requires an estimate of the gain value. This gain estimate takes time, and is usually noisy if limited time is available, so in applications where limited time is available (as in IEEE 802.11a) a poor estimate must be used, resulting in a noisy correlator output and an increased probability of false detection.
The poor gain estimate means that a conventional normalization will create a correlator output that is either too large or too small depending on the difference between the magnitude of the input signal and the value of the gain estimate.